1. Field of the Invention
The present invention relates to an impact resistant structure for a helicopter and an energy absorber used for the same.
2. Description of the Related Art
A helicopter is often operated in visual flight rule, or between mountains, or at a low altitude, because of its operating characteristics. Then, there is always a risk of accident due to contact with an obstacle. Therefore, an impact resistance is strongly required in a helicopter in order to keep survivability of crew members in the crash situations.
The basic principle for the impact resistant structure of a helicopter is to adopt a continuous strong keel K for a nose H easily crushed and a bottom G easily crushed which are shown in FIG. 16(a) to prevent a floor D from structural failure on crash landing as shown in FIG. 16(b), to adopt a strong outer skin P as shown in FIG. 16(c), to adopt a strong beam B on the keel K, and to adopt a continuous strong frame F.
For a helicopter of which landing gear, such as retracted one, may not be effectively functioned for crash energy absorption, an impact resistant fuselage structure having impact absorption capacity is required for the typical crush environment shown in FIG. 17 in a shape fitting to the actual helicopter fuselage structure.
Conventionally, the floor structure of a helicopter is designed according to a normal operational flight load and a landing load on the ground. At present, general impact absorption to an unexpected crash impact like crushing shown in FIG. 17 is not taken into account.
Conventional impact resistant structures for the helicopter are disclosed in U.S. Pat. No. 4,593,870, U.S. Pat. No. 5,069,318, and U.S. Pat. No. 5,024,399. Meanwhile, in a helicopter, on the typical ground surface, as shown in FIG. 18, the ground reaction force is concentrated on the outer wall, though in the impact resistant structures disclosed in the above-mentioned U.S. patents, a floor member is not arranged so as to be suitable to ground reaction force. Further, as shown in FIG. 19 (Ref. “Full-Scale Crash Test of the Sikorsky Advanced Composite Airframe Program Helicopter” Richard L. Botinott, AHS 56th), the web intersection part X is hard to be crushed, and the sub-floor effective stroke is not effectively utilized, so that a sufficient floor acceleration reduction is not realized. Furthermore, the effective function, under the condition in which the landing gear is not effectively functioned, against the combined crash speed environment of the horizontal speed and drop speed of the general crash environment of a helicopter shown in FIG. 17 is not disclosed in the U.S. patents.
Examples of impact resistance absorption members used in the impact resistant structure of a helicopter and the impact resistant structure for general industrial purpose are disclosed in Japanese Patent Laid-Open Publications No.2002-286066, No.2002-36413, No.2001-153169, No.2000-192432, and U.S. Pat. No. 5,746,537, such as an example using axial compression energy absorption of a light weight fiber reinforced composite material tube, and an example that a foaming material is filled up in all sections for energy absorption improvement.
However, to reduce an impact load by a long absorption stroke, and simultaneously, to realize high impact energy absorption of the merit of a fiber reinforced composite material tube without instability of overall general buckling, if the section of the single tube is simply made larger, the local buckling tendency of the tube wall will be increased and the stable progressive failure mode suitable for impact energy absorption of fiber reinforced composite tube shown in FIG. 20 cannot be achieved. Further, when a foaming material is filled in all the sections, a space for releasing destroyed small pieces of the composite material generated in the progressive failure mode is lost, and the destroyed small pieces are compacted, and the energy absorber will become extremely stiffened. Thereby, the effective stroke is reduced, and a required impact absorption capability is not obtained. Further, to reduce the local unstable buckling of the tube wall, when the section size of the tube is made compact, the aspect ratio (energy absorber height/section width) of whole the energy absorber becomes long and slim, and the energy absorber becomes weak for bending and eccentric compression, so a desired axial compression energy absorption property cannot be achieved. As a solution for these problems, when the section of the tube is made compact, and is made into a bundling shape, when the number of tubes is optionally adjusted, the number of intersections between stiff walls increases as the number of tubes increases, so that as shown in FIG. 21, the initial load peak level harmful for the impact absorption property increases.